Two problems that occur in communication systems are referred to as dropout and fade. These problems arise from a condition known as multi-path, wherein a signal proceeds to a receiver along a plurality of different transmission paths. As a result, the net received signal is a composite of a number of signals having different and randomly varying delays and amplitudes.
One approach to dealing with multi-path conditions is described in a journal article entitled "A Communication Technique For Multi-Path Channels" by R. Price and P. E. Green, Jr. (proceedings of the IRE, March 1958, pages 555-570). These authors describe a system that performs a continuous, detailed measurement of a multi-path characteristic. This knowledge is exploited to combat the effects of multi-path. Particularly, selective fading is treated by detecting the echo signals individually, using a correlation method, and adding the signals algebraically, rather than vectorially. Intersymbol interference is dealt with by reinserting different delays into the various detected echoes. This system is referred to as a "Rake" system.
In general, the purpose of the Rake system is to equalize the multi-path dispersion from a single transmitter through an ionospheric multi-path medium, thereby reducing the intersymbol interference and improving the signal-to-noise ratio (SNR).
The Rake system is applicable to a large number of small incremental delays resulting from the ionospheric multi-path, and therefore employs lengthy tapped delay lines, having a large number of taps, to compensate for the large numbers of multiple paths. Furthermore, the Rake signals have no significant Doppler spread, and the length of the tapped delay line is significantly shorter than the length of an information bit.
As a result, the Rake system is not specifically adapted for use with combinations of received signals having both a large time and frequency spread.
In a journal article entitled "Increased Capacity Using CDMA For Mobile Satellite Communication", by K. S. Gilhousen et al. (IEEE Journal on Selected Areas In Communications, vol. 8, No. 4, May 1990, pages 503-514) there is described the performance of a spread-spectrum (SS) Code Division Multiple Access (CDMA) communication system in a mobile satellite environment. These authors mention at page 506 that multiple satellites provide a further technique of improving the CDMA capacity, and that a possibility exists for an additional capacity increase by the coherent combining of signals transmitted between a terminal and all satellites in view. The use of coherent combining is said will result in a capacity gain approaching the increased number of satellites.
In U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters", K. S. Gilhousen et al. describe a communication system that provides marginal isolation between user communication signals. The marginal isolation is provided by (a) generating simultaneous multiple steerable beams, using an omnidirectional antenna with polarization enhancement, and (b) using power control devices to adjust the output power for user generated communication signals, either in response to their input activity level, or in accordance with a minimum allowable power for maintaining a communication link. In col. 32 it is stated that the repeaters for the communication links are orbital or terrestrially based repeater stations that can provide a variety of communication paths to compensate for Doppler Shifts and multi-path problems.
What is not disclosed by the foregoing, and what is thus an object of this invention to provide, is a construction for a spread spectrum radio frequency receiver so as to provide substantially fade free communications between a transmitter and the "receiver, via a plurality of linear communications repeaters.